Mobile Switching Centre, MSC, in pool is a concept whereby a Radio Access Network node (RAN node), which can be for example RNC (Radio Network Controller) or BSC (Base Stations Controller) is controlled by a pool of MSCs. This is intended to provide more flexibility in extending a mobile core network with additional MSCs, reduction in signaling in the core network and network redundancy. In such configuration if one MSC in the pool experiences a system failure the RAN nodes continue to be controlled by the remaining MSCs in the pool. Without MSC in pool a RAN node may only be controlled by a single MSC and a failure of this MSC results in disruption of services provided to subscribers served by the affected RAN nodes.
FIG. 1 illustrates a telecommunications network 100, in which MSCs are organized in pools. Pool 1 consists of MSC 1, MSC 2 and MSC 3. It controls all the RAN nodes (these may be BSCs or RNCs) in pool area 1—RAN node 1, node 2, node 5 and node 6. Pool 2 consists of MSC 4, MSC 5 and MSC 6 and it controls all the RAN nodes in pool area 2, i.e. is RAN node 3 and node 7. MSC 7 is not in any pool and it is the only MSC controlling RAN node 4 and node 8. When a mobile subscriber moves into the pool area controlled by the MSC pool the RAN node decides which MSC to send the registration request to.
If an MSC in pool 1, for example MSC 2, experiences a complete failure the remaining MSCs in pool 1, i.e. MSC 1 and MSC 3, will continue to control all the RAN nodes in pool areal thus avoiding loss of service to the mobile subscribers server by the RAN nodes in pool area 1.
3GPP does not specify a particular method for load balancing. However a typical situation is that a relative weighting is assigned to each MSC in the pool. Non Access Stratum (NAS) Node Selection Function in a RAN Node will allocate subscribers to an MSC in the pool on the basis of this weighting.
Overload may occur due to geographical factors such as presence of major roads and commuter patterns, MSCs managing RAN Nodes which are outside of the pool area as well as due to the number of RAN nodes in the pool area and processing capacity of the MSC.
The example given above is applicable not only to Mobile Switching Centres operating in a pool, but also to Serving GPRS Support Nodes operating in a pool.